9.2. NFS Server Configuration Files

Configuring a system to share files and directories using NFS is a simple process. Every file system being exported to remote users via NFS, as well as the access rights relating to those file systems, is located in the /etc/exports file. This file is read by the exportfs command to give rpc.mountd and rpc.nfsd the information necessary to allow the remote mounting of a file system by an authorized host.

The exportfs command allows the root user to selectively export or unexport directories without restarting the NFS service. When exportfs is passed the proper options, the file systems to be exported are written to /var/lib/nfs/xtab. Since rpc.mountd refers to the xtab file when deciding access privileges to a file system, changes to the list of exported file systems take effect immediately.

Various options are available when using exportfs:

If no options are passed to the exportfs command, it displays a list of currently exported file systems.

Changes to /etc/exports can also be read by reloading the NFS service with the service nfs reload command. This keeps the NFS daemons running while re-exporting the /etc/exports file.

9.2.1. /etc/exports

The /etc/exports file controls which file systems are exported to remote hosts and specifies options. Blank lines are ignored, comments can be made by starting a line with the hash mark (#), and long lines can be wrapped with a backslash (\). Each exported file system should be on its own individual line. Lists of authorized hosts placed after an exported file system must be separated by space characters. Options for each of the hosts must be placed in parentheses directly after the host identifier, without any spaces separating the host and the first parenthesis.

In its simplest form, /etc/exports only needs to know the directory to be exported and the hosts permitted to use it:

/some/directory bob.example.com

After re-exporting /etc/exports with the /sbin/service nfs reload command, the bob.example.com host will be able to mount /some/directory and can mount /another/exported/directory. Because no options are specified in this example, several default NFS preferences take effect:

In order to override these defaults, you must specify an option that takes its place. For example, if you do not specify rw, then that export will only be shared read-only. Each default for every exported file system must be explicitly overridden. Additionally, other options are available where no default value is in place. These include the ability to disable sub-tree checking, allow access from insecure ports, and allow insecure file locks (necessary for certain early NFS client implementations). See the exports man page for details on these lesser used options.

When specifying hostnames, use the following methods:


The format of the /etc/exports file is very precise, particularly in regards to use of the space character. Remember to always separate exported file systems from hosts and hosts from one another with a space character. However, there should be no other space characters in the file except on comment lines.

For example, the following two lines do not mean the same thing:

/home bob.example.com(rw)
/home bob.example.com (rw)

The first line allows only users from bob.example.com read-write access to the /home directory. The second line allows users from bob.example.com to mount the directory read-only (the default), but the rest of the world can mount it read-write.